1. Field of the Invention
The invention relates generally to semiconductor packaging. More specifically, the invention relates to the design and manufacturing process of a semiconductor package that allows the incorporation of more than one chip device into a single package.
2. Description of the Related Art
For a variety of reasons well known in the art, semiconductor packages that have more than one chip, otherwise known as multi-chip modules (xe2x80x9cMCM""sxe2x80x9d), are becoming increasingly popular. One type of possible MCM is shown in FIG. 1, which is disclosed in U.S. patent application Ser. No. 09/461,704, entitled xe2x80x9cMulti-Chip Package Including Memory and Logic,xe2x80x9d filed Dec. 14, 1999, and is owned by the assignee of the present invention. The MCM shown in FIG. 1 comprises a top die 10 stacked on top of a bottom die 12, which is in turn mounted on a substrate 18. The top die 10 is mounted on the bottom die 12 in flip chip fashion such that the top die 10 communicates with the bottom die 12 through a plurality of (collapsed) solder balls 20. The bottom die 12, in turn, communicates with the substrate 18 through a plurality of wires 14. Thus, the bottom die 12 has two types of bond pads: flip-chip bond pads that are coupled to a plurality of solder balls 20 and wire bond pads that are coupled to the plurality of wires 14.
Both types of bond pads are aluminum. Aluminum bond pads form effective bonds with the plurality of wires 14. However, the aluminum bond pads may not effectively bond to the plurality of solder balls 20, which often comprise eutectic solder (e.g., 63% Sn/37% Pb). The mismatch between the different types of bonding, solder ball and wire bond, makes it somewhat difficult to manufacture the package shown in FIG. 1. It would be desirable to improve the efficiency of the manufacturing process for a package such as that shown in FIG. 1.
To achieve the foregoing, the present invention provides a method for manufacturing a die comprising two different types of bond pads, one type that is compatible with wire bonding and another type that is compatible with solder bonding. In one embodiment, the bond pads that are compatible with wire bonding have a top layer of aluminum while the bond pads that are compatible with solder bonding have a top layer of copper.
All of the bond pads can initially consist of aluminum. To selectively form copper over the solder compatible bond pads, a layer of copper is disposed over all the bond pads and selectively etched away from over the wire bonding compatible bond pads. The copper can be selectively etched by disposing a layer of photoresist over the copper, removing the photoresist from over the wire bonding compatible bond pads, and then dissolving the copper in a suitable solution. The solution, dissolved copper and remaining photoresist are then removed, thereby leaving copper over the solder compatible bond pads while returning the wire bonding compatible bond pads to aluminum without an overlay of copper.